Gas turbine engines and other types of machinery are subject to vibration which can loosen nuts and bolts. Safety wire is often used as backup protection in order to prevent the loosening. Safety wiring is the securing together of two or more parts with wire such that any tendency to loosen will be counteracted by an additional tightening of the wire.
Several types of safety wire are shown in FIGS. 1A-1E. Several aspects of these wires should be noted. One, the wire should be positioned such that loosening of a bolt causes an increase, rather than a decrease, in tension of the wire. The increasing tension opposes further loosening. That is, assuming that counterclockwise rotation of bolt 3 in FIG. 1A loosens the bolt, then such rotation tightens wire pair 5, and the tightening inhibits further rotation and loosening of the bolt. However, if the configuration is that of FIG. 1E, the same rotation loosens the wire as indicated by phantom wire 6. This latter configuration must be avoided.
A second aspect is that the direction (clockwise or counterclockwise) of wire twisting must be proper so that the wire section 9 in FIG. 1B which skirts the bolt 3 cannot unravel. That is, for example, the skirting section 9 is part of a counterclockwise-twisted section 11 (counterclockwise direction is indicated by arrow 12.) With this counterclockwise twisting, the skirting section 9 is pinned against rotation by the other wire 18. In contrast, skirting section 21 in FIG. 1B is not pinned in the same manner, and this latter skirting section 21 can rotate into position 25, thus loosening tension on the twisted wire pair 27. Thus, it is required that a skirting section be properly pinned by its companion wire.
A third aspect is that the number of twists per running inch of twisted wire must be held within a predetermined range.
A fourth aspect is that the twisted wire must have a tension which is within acceptable limits. The tension is commonly measured as shown in FIG. 2, wherein deflection 34 of the twisted wire 5 into position 36 under a specified force 37 must not exceed a certain maximum.
A fifth aspect is that the maximum gap 39 between a bolt head 3 in FIG. 1A and the termination point of wire twisting must not exceed a limit, such as one-eighth inch.
A sixth aspect is that the pigtail 41 (i.e., the final length of the locking wire trailing from the last bolt which is fastened) must not exceed a certain length, must have a certain number of twists, and must be bent toward the bolt to assure that the cut ends are not exposed to cause snags.
A seventh aspect is that excessive nicks and kinks in the wire are to be avoided.
In order to properly install a safety wire in all of the preceding aspects, sometimes several attempts are necessary because of the difficulty involved. Further, sometimes a subjective judgement is involved as to whether a given installation is proper. For example, there is no generally agreed standard as to what nicks and kinks are acceptable. Therefore, some quality control inspectors may fail a given wire installation, while others may pass the same installation.
The two facts that (1) the difficulty of the installation requires multiple attempts and (2) some installations are rejected by inspectors when, perhaps, they should not be, cause increased cost in assembling the safety wire installations.
Further information concerning industry practices regarding safety wiring can be found in Aerospace Standard AS567, entitled "General Practices for the use of Lockwire, Key Washers, and Cotter Pins," available from the Society of Automotive Engineers, Inc., 400 Commonwealth Drive, Warrendale, Pa.